RO membranes have been used in an ultra pure water manufacturing plant, a drainage recovery plant, a seawater desalination plant, and the like, and by a treatment using an RO membrane, organic substances, inorganic substances, and the like in water can be mostly removed. However, because of influences of oxidizing materials and/or reducing materials present in water, acid/alkali cleaning, aging degradation, and the like, the rejection rate of an RO membrane is gradually decreased, and as a result, required qualities of permeated water (treated water) may not be obtained in some cases. In addition, the removal performance may be degraded in some cases by an unexpected trouble, and the rejection rate itself of an RO membrane as a finished product may not satisfy a required level in some cases.
In a water treatment system, in order to prevent propagation of slime in the system, sodium hypochlorite or the like is added thereto in many cases. Since chlorine is a strong oxidizing agent, when an aqueous solution is supplied to an RO membrane while residual chlorine is not sufficiently removed by reduction, chlorine and the RO membrane are brought into contact with each other, and the RO membrane is degraded thereby.
In order to decompose residual chlorine, although a reducing agent, such as sodium bisulfite, may be added in some cases, under a reducing environment in which sodium bisulfite is excessively added, when a heavy metal, such as Cu, Co, Mn, and/or Fe, is also present therewith, the membrane is degraded (Patent Literature 1, Non Patent Literature 1).
As a method for improving a rejection rate of a permeation membrane, such as an RO membrane, the following methods have been proposed.    (1) A method in which the rejection rate is improved by adhering an anionic or a cationic high molecular compound to a membrane surface (Patent Literature 2).    (2) A method in which the rejection rate of a nano-filtration membrane or an RO membrane is improved by adhering a compound having a poly(alkylene glycol) chain to a membrane surface (Patent Literature 3).    (3) A method in which the salt rejection rate is improved by adhering a tannic acid or the like to a degraded membrane (Non Patent Literature 2).
However, according to the related methods described above, there has been a problem in that since a decrease in permeation flux (flux) is large, even if the rejection rate is restored, the permeation flux is decreased by 20% or more of that obtained before the treatment in some cases or a problem in that a seriously degraded membrane is difficult to be sufficiently restored.
As a method to overcome the above problems of the related techniques, the present applicant has already proposed a method for improving a rejection rate which is characterized in that an aqueous solution containing a first organic compound having a molecular weight of less than 200, a second organic compound having a molecular weight of 200 to less than 500, and a third organic compound having a molecular weight of 500 or more is allowed to pass through an RO membrane (Patent Literature 4).
However, in the method disclosed in Patent Literature 4 for improving a rejection rate using a polyphenol, such as a tannic acid, as the third organic compound having a molecular weight of 500 or more, a problem in that the rejection rate and the flux are gradually decreased by raw water quality occurs; hence, a method which further improves the stability of removal performance (rejection rate) and flux stability (sustainability of contamination resistance) has been desired.